Much information concerning the nature of subterranean formations is obtainable by analyzing signals or waves produced by a seismic disturbance. Such disturbances are artificially produced by utilization of a variety of energy sources such as dynamite or other high energy explosive, implosive air guns, electrical sparkers, and vibrating chirp systems. The energy source generates seismic source waves that propagate through the earth and are reflected and refracted from subterranean interfaces where the acoustic velocity and density are sufficiently different between adjacent materials. The waves eventually return to the earth surface where they are detected by a selected array of seismometers such as geophones (for land use) and piezoelectric transducers (for marine use), which are distributed along the earth surface above the area of exploration. The seismometers convert the siesmic mechanical vibrations into electrical signals. By studying the electrical signals produced by the seismometers in response to the seismic wave vibrations and by knowing other variables such as the location of the seismometers and the location of the seismic source wave generator, analysts are able to make determinations concerning the nature of the subterranean structure of the earth.
Many types of seismic disturbances possess a broad frequency spectrum that extends from a few hertz (cycles per second) to several kilohertz (thousands of cycles per second). As is well known to geophysicists, seismic waves in the upper end of the seismic spectrum become attenuated (weakened) as the waves travel deeper into the earth. The energy loss is due to solid friction in the earth and to scattering by inhomogeneities such as boulders and fractures in the subsurface strata. Accordingly, seismic energy from relatively shallow strata, 500 to 1000 feet deep, may be rich in high frequencies up to 1 or 2 kilohertz whereas seismic energy reflected from deep-seated strata is characterized by much lower frequencies in the 10-30 hertz range.
In seismic exploration, the stratigraphic resolution (the capability of distinguishing two closely spaced strata or earth layers) depends, among other things, upon the frequency of the reflected seismic signals. Use of relatively high-frequency reflections in the range of 100 to 1000 Hz (hertz) is needed to distinguish between layers that are separated by only a few feet. Stratigraphic resolution of the above-defined degree is required for study of shallow strata in connection with engineering foundation problems, coal exploration, etc. In petroleum exploration, higher resolution is sought at all levels but is more critical at the shallower levels because smaller deposits may be of interest in view of the lower cost of drilling shallower oil wells. Grosser resolution of deep-seated strata may be found to be acceptable. Hence, lower-frequency reflections can be useful for deeper exploration.
In conventional practice, the electrical signal produced by the seismometer, in response to the seismic wave vibrations, is sampled at selected time intervals, converted to digital form such as binary numbers, and recorded on magnetic tape. At some later time, the original electrical signal is reconstructed from the tape-recorded data samples. In accordance with the well known Nyquist theorum, the sampling rate must be at least twice as fast as the highest frequency which is to be transmitted or converted. To attenuate frequencies higher than the Nyquist frequency, it is customary for many signal processing systems to include "alias" filters.
The period of recording electrical signals transmitted by the seismometers normally lasts between 6 and 15 seconds after the generation of the seismic source waves, depending upon the depth of the deepest subterranean structure of interest and upon the type of energy source utilized. The amplitude of the electrical signal may vary 100 or more decibels during the recording period. Also, during the first few seconds of recording, the electrical signal results generally from the waves reflected, etc., from shallow formations, whereas, during the last few seconds of recording, the electrical signal results generally from waves reflected, etc., from deep formations. Thus, the electrical signal is initially composed of both high and low frequencies, and is later composed of lower frequencies.
A patentability search was conducted for the present invention, and the following patents were uncovered:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,414,874 McLoad Dec. 3, 1968 3,491,848 Giles Jan. 27, 1970 3,840,845 Brown Oct. 8, 1974 3,852,708 Doolittle et al. Dec. 3, 1974 3,979,713 Parrack Sept. 7, 1976 4,042,905 Fort et al. Aug. 16, 1977 4,146,870 Ruehle Mar. 27, 1979 4,187,492 Delignieres Feb. 5, 1980 ______________________________________
U.S. Pat. No. 4,042,905 to Fort et al. states that in prior art seismic methods, it has been customary to use a large plurality of separate groups of geophones each connected by a two conductor cable which carries the geophone signals to a distant recording truck. At the truck each of the separate signals from each of the separate pairs of conductors is separately amplified, controlled, multiplexed and converted from analog to digital form for storage on a magnetic medium.
The other patents discovered during the patentability search are believed to have virtually no relevance to the present invention and serve merely to illustrate the state of the art of seismic surveying systems.
U.S. Pat. No. 3,867,713 to Tegland et al. relates to a seismic survey exploration method using a common depth point technique. Again, this patent is believed to be irrelevant to the present invention other than for showing the state of the art.
An article entitled "Seismic Signal Processing" by Lawrence C. Wood and Sven Treitel and appearing in the proceedings of the IEEE, Volume 63, No. 4 April, 1975 discusses the then prevailing state of the art techniques used in seismic surveying. This article is also believed to be irrelevant to the present invention other than for showing background art.
U.S. Pat. No. 4,041,443 to Thigpen discloses a seismic recording apparatus having a time-varying signal sample rate. The apparatus includes several signal processing channels and means for switching the signal through any one selected channel during the signal recording period. Each channel includes an alias filter, an amplifier, an analog to digital converter and a specified signal sampling rate. Among the drawbacks of the apparatus is that the signal switching causes signal instability and distortion which is especially significant during relatively fast sample rates. Moreover, at any one instant, a relatively wide bandwidth of the signal is processed and analyzed, and it is impossible to optimize the processing of such a relatively broad bandwith signal over the entire frequency range.